Is there such a function in PHP that does grep -f filename in unix/linux systems. If there is none, what PHP functions/tools would help in creating a customised method/function for this. Thanks!
Actually IMHO, I'd say it's the following:
$result = preg_grep($pattern, file($path));
See preg_grepDocs and fileDocs.
If you need to do that (recursively) over a set of files, there is also glob and foreach or the (Recursive)DirectoryIterator or the GlobIteratorDocs and not to forget the RegexIteratorDocs.
Example with SplFileObject and RegexIterator:
$stream = new SplFileObject($file); 
$grepped = new RegexIterator($stream, $pattern); 
foreach ($grepped as $line) {
    echo $line;
}
Output (all lines of $file containing $pattern):
$grepped = new RegexIterator($stream, $pattern); 
foreach ($grepped as $line) {
    echo $line;
}
Demo: https://eval.in/208699
You can combine the file_get_contens() function to open a file and the preg_match_all() function to capture contents using a regex.
$file = file_get_contents ('path/to/file.ext');
preg_match_all ('regex_pattern_here', $file, $matches);
print_r ($matches);
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